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Engine Systems Slide 2 Introduction Engines permeate our society. Labor costs for small engine are over $50.00 an hour. To operate a small engine efficiency you must be able to troubleshoot engine problems. To be a good troubleshooter you must know the name and function of all of the engine parts. 2 Slide 3 3 Five (5) Engine Systems All engine parts and functions can be divided into five (5) systems. 1.Compression 2.Fuel 3.Electrical 4.Cooling 5.Lubrication If an engine will not start, which one of the five system(s) is most likely the problem? If an engine has a lot of operating time, is hard to start and doesnt seem to be producing as much power as it once did, which one of the five system(s) is most likely the problem? Slide 4 4 Compression System The compression system includes all of the parts that create, contain and manage the engine compression. Block Piston Piston rings Cylinder head Bearings Parts Valves Valve springs Connecting rod Crankshaft Gaskets Slide 5 5 Compression Problems Two possible problems: Inadequate compression SymptomsCause(s) Leaks Poor starting Less power Excessive compression Symptoms Harder to crank Detonation Preignition Cause(s) Carbon buildup in combustion chamber Slide 6 6 Detonation An undesirable engine condition in which pockets of fuel start to burn at about the same time as the spark plug fires. Multiple pressure fronts collide. Sometimes called knocking, spark knock or pinging. Causes large pressure differentials in the combustion chamber. Will cause engine damage. Slide 7 Detonation 7 Causes 1.Increased compression 2.High temperatures 3.Lean fuel/air mixture 4.Advanced ignition timing 5.Low octane fuels How do you prevent detonation? Slide 8 8 Preignition Fuel starts to burn before the spark plug fires. Increases the peak combustion pressure in the cylinder. Increases internal temperature. Decreases engine performance and produces and audible pinging or knocking sound in the engine. Will cause engine pistons, connecting rods, crankshafts and other compression system parts to fail. Slide 9 Preignition 9 Causes 1.An overheated spark plug 2.Glowing carbon deposits 3.Over heated exhaust valve 4.A sharp edge in the combustion chamber or on top of the piston 5.Sharp edges on valves that were reground improperly 6.A lean fuel mixture. How do you prevent preignition? Slide 10 10 Piston Rings Wiper ring Meters oil film on cylinder walls Must be installed correctly. Oil ring Constructed of two thin rails with holes or slots cut in-between. Has the highest pressure against the cylinder wall of the three rings. Compression Subject to to greatest amount of chemical corrosion and highest temperatures. Transfers 70% of combustion heat from piston to cylinder walls. Compression Ring Wiper Ring Oil Ring Slide 11 11 Crankcase Breather Maintains pressure in the crankcase at less than ambient pressure to assist in the control of oil consumption. Excessive blow by renders the breather useless. Old engines vent to the atmosphere. New engines vent to the carburetor. Should the crankcase breather be removed during regular servicing? Slide 12 12 Compression Release Compression release systems are used to decrease effort required to start engine. Holds the exhaust valve slightly open during starting, and then allows it to fully close once engine starts. Compression release pin Slide 13 Fuel System Slide 14 14 Introduction The function of the fuel system includes: 1storing 2metering 3atomizing 4vaporizing 5mixing fuel and air 6delivering fuel air mixture to intake manifold What symptom(s) will an engine exhibit if the fuel system has failed? Slide 15 Engines Fuels 15 Common Small Engine Fuels include: Gasoline Diesel LPG LNG CNG Why are so many different types of fuel available? Slide 16 Fuel systemcont. Fuel system parts: Supply (tank) Lines Shut off valves Filter Pump Carburetor 16 What is the function of each part? Slide 17 Additional facts about the fuel system: Fuel must be clean and good quality Air fuel mixture must be richer for starting Excess fuel washes lubrication off of the cylinder walls and accelerates wear. Excess fuel dilutes the oil and accelerates wear. Air fuel mixture that is too lean will cause preignition 17 Fuel systemcont. Slide 18 18 Volatility Volatility is the propensity of a liquid to become a vapor. The volatility of gasoline changes with the seasons. Low Volatility (summer) High Volatility (winter) Poor cold weather operationPoor hot weather operation Spark plug deposit buildupVapor lock Combustion chamber deposit buildup Poor fuel economy Poor cold startingExcessive fuel evaporation What is vapor lock? Slide 19 19 Vaporization Vaporization is the process converting a liquid to a vapor. Fuel must be vaporized because liquid fuel will not burn. The rate and efficiency of vaporization is improved when the liquid is reduced to small droplets (atomized). What is required to vaporize a liquid fuel? Slide 20 20 Carburetor Slide 21 21 Carburetor: the engine component that provides the required air-fuel mixture to the combustion chamber based on engine speed and load. Identify the parts on the carburetor and explain their function(s)? E G D C B F H A Newer carburetor What is different? Slide 22 22 Four Fluid Principles 1.Fluids flow from areas of high pressure to areas of low pressure. 2.When there is no pressure difference-- there is no fluid flow. 3.Fluids exert pressure of the same value throughout a system. 4.Fluid flow in a carburetor utilizes Bernoullis principle. Air flowing through a narrowed portion of a tube (Venturi) increases in velocity and decreases in pressure. Name another application of Bernoullis principle. Slide 23 23 Carburetor Operating Principles A carburetor is a tube attached to the intake port of the engine and open to the atmosphere. On the intake stroke a volume with little to no pressure develops in the combustion chamber. Atmospheric pressure outside the engine--14.7 psi Low pressure in the combustion chamber--0 to slight vacuum. Result: air wants to flows from outside of to inside the engine. What can reduce the amount or rate of air flow? Slide 24 24 Carburetor Operating Principles As the air flows through the carburetor, the fuel is metered, atomized and vaporized. To have available fuel, the carburetor must have a source of fuel. In the float type carburetor this source is the fuel bowel. Why does the carburetor have an air vent to the fuel bowel? Slide 25 25 Carburetor--Venturi A fluid will not flow without a pressure difference A pressure difference is needed to cause the fuel to flow from the fuel bowel into the air stream. This is accomplished using a Venturi and a tube connecting the mouth of the venture to the fuel bowel. Add a jet to the emulsion tube and you have a functioning carburetor. It will function correctly as long as it has a constant load and constant speed. How many engines operate at a constant load and constant speed? Slide 26 26 Carburetor-Throttle To adjust the rate of fuel flow a throttle is used. When the throttle is in the closed position there is minimum air flow through the carburetor. Low air flow = less pressure difference between the venture and fuel bowel less pressure difference = less fuel flow less fuel flow = less speed. What happens when the throttle is opened?Why? Slide 27 27 Throttle--cont. When the throttle is in the wide open position, there is maximum air flow through the carburetor. Maximum air flow = maximum pressure difference between Venturi and fuel bowel Maximum pressure difference = maximum fuel flow Maximum fuel flow = maximum speed & power. A carburetor with this design would function well under varying loads and speeds, Older carburetors used a needle valve to adjust the fuel flow into the emulsion tube. Newer carburetors use a fixed orifice. What controls the position of the throttle? Slide 28 28 Carburetor-Choke Engines require a richer mixture for starting. This is accomplished by a choke or primer bulb. When the choke is closed the maximum pressure difference occurs in the Venturi. Maximum pressure = maximum fuel flow. Once engine starts the choke must be opened to prevent the engine from running too rich. A primer bulb is used on some engines. Slide 29 Carburetor-Chokecont. Briggs and Stratton newest engine uses a thermostatically controlled choke. An air vane is attached to the choke lever and the lever on the choke control. As the bimetallic strip in the thermostat heats up it moves the lever and the air vane opens the choke. 29 Slide 30 30 Carburetor-Idle Circuit The addition of a choke/primer improved engine starting, but this carburetor still has a problem if the engine needs to run at idle. When the throttle is in the idle position, almost closed, the area with greatest restriction, and greatest pressure difference, moves from the Venturi to the area between the throttle plate and the carburetor wall. No low pressure area in the Venturi means no fuel flow. This problem was solved with the addition of an idle circuit and idle needle valve. Many new walk behind mowers are single speed. They are either running at full governed speed or off? Will these carburetors have an idle circuit. Slide 31 31 Carburetor-Float To have constant fuel flow with constant pressure difference, the lift, distance from the top of the fuel to the top of the main nozzle, must remain the same. A constant level of fuel is maintained in the fuel bowel by the float, float needle valve and float needle valve seat. Slide 32 32 Carburetor-Additional Features Several additional features have been tried/added to improve carburetor performance. Air bleeds Fixed jets Transition ports Pilot jets Slide 33 33 Three Types of Briggs & Stratton Carburetors Vacu-jet Carburetor mounted on top of fuel tank. Must use shallow fuel tank because the main jet extends from the Venturi to the bottom of the fuel tank. As the level of fuel in the tank changes, the fuel-air ratio changes. Not included in latest B & S repair manual. Slide 34 34 Three Types of Briggs & Stratton Carburetors--cont. Pulsa-jet Carburetor mounted on top of fuel tank. Carburetor includes a fuel pump that elevates fuel from the main tank to the secondary tank. The combination of excess fuel pump capacity and secondary tank drain keeps the fuel level in the secondary tank at a constant level. Not included in latest B & S repair manual. Slide 35 35 Pulsa Jet Parts A.Fuel pump B.Primary fuel tube C.Primary fuel tank D.Primary fuel tube check valve E.Fuel screens F.Secondary fuel tube check valve G.Secondary fuel tank H.Secondary fuel tube I.Choke J.High speed needle valve K.Air horn (inlet) Slide 36 36 Three Types of Briggs & Stratton Carburetors--cont. Flow-jet Different types and sizes are used. Most popular on modern engines. All use a fuel bowel and float system to maintain a consistent supply of fuel. Slide 37 Governor System Slide 38 38 Introduction The function of the governor system is to maintain the desired engine speed regardless of engine load. The governor is attached to the throttle on the carburetor and supplies a force that attempts to close the throttle. The governor spring is attached to the governor linkage and applies a force that attempts to open the throttle. A constant engine speed means these two forces are balanced. Slide 39 39 Mechanical Governor The weights are rotated by the governor gear which meshes with the crankshaft gear. As the governor spins the governor weights move out from the center shaft. The weights are mounted on a lever arm that pushes the governor shaft up as the weights move out. The higher the speed the greater the force produced. What will be the effect if the governor doesnt work, or because of slack in the linkage the throttle plate doesnt move? Slide 40 Mechanical Governor The governor spring is constantly trying to open the throttle and the governor is always trying to close the throttle. 40 Slide 41 Electrical System Slide 42 42 Introduction Electricity is a predictable force, yet it is often challenging to service electrical systems because it can not been seen and there is the concern of electrical shock. Because almost all small engine electrical systems operate on 12 volts, the danger of severe electrical shock is reduced. Slide 43 43 Electrical Terms Before attempting to understand small engine electrical systems, it is important to know the terms and parts associated with electricity and the electrical systems. Electricity Conductor Electron Free Electron Voltage Load Current Direct Current Alternating Current Polarity Amperes Resistance Short circuit Series Circuits Parallel Circuits Ohms Law Magnetism Induction Solenoid Diodes Voltage Regulator Battery Primary winding Secondary winding Condenser Slide 44 44 Resistance Resistance is opposition to the flow of electrons. All circuit components have some resistance. Forcing electricity through a resistance uses energy. The energy is lost as heat. Resistance is measured in units of Ohms ( ). The amount of current flow and resistance in a circuit determines the wire size for the circuit. AWG NumberDiameter /1000 ft (68 o F) 1280.81.6 1464.12.5 1650.84.0 1840.36.4 203210.2 2225.3516.2 Wire Size and Resistance Slide 45 45 Magnetism Magnetism is an atomic level force derived from the atomic structure and motion of certain orbiting electrons. A Magnet field is an area of magnetic force created and defined by lines of magnetic flux surrounding a material in three dimensions. Magnetic flux: invisible lines of force in a magnetic field. Magnet: a material that attracts iron, cobalt or nickel and produces a magnetic field. Permanent Temporary Slide 46 46 Induction Induction: the production of voltage and current by the proximity and motion of a magnetic field or electric charge. With a conductor, either current, a magnetic field or motion can be produced as long as the other two are present. Magnetic field: When electricity passes through a conductor it forms a magnetic field around the conductor. Current: When a conductor passes through a magnetic field or when magnetic field moves and/or varies in strength around a conductor, electrons are made to flow. A current is induced in the conductor. Slide 47 47 Small Gas Engine Electrical Systems Small engines may have one or more of five (5) electrical systems. 1.Charging 2.Ignition 3.Starting 4.Accessories 5.Safety Slide 48 48 1. Charging System Charging systems produces electrical to operate accessories and the replace electrical energy taken from a battery. Two different systems can be used. Generator Alternator Generator produces DC. Alternator produces AC. When DC is needed the current is converted, rectified. Some small engines use a stationary coil close to the flywheel. When the flywheel magnets pass by the coil they induce a current in the coil. Other systems use stationary magnets and a rotating coil. Conductors are sized for circuits with low current flow. Slide 49 49 Charging System--cont. The components of a charging system may include: Coil Magnets Voltage regulator Rectifier Switches Conductors Slide 50 50 2. Ignition System The ignition system provides a high voltage spark in the combustion chamber at the proper time. Two types of ignition systems Battery Magneto Battery Battery systems transforms the battery voltage and fires the spark plug at the correct time. Magneto Magneto systems must produce the current, transform the voltage and time the spark plug. Most small engines use the magneto system Two types of magneto systems: Breaker point ignition Solid state (electronic) ignition Slide 51 51 2. Ignition System-cont. Breaker point ignition Older system. Most manufacturers have replaced them with solid state. Uses a set of points to break the primary circuit. Solid state ignition Uses a transistor to break the primary circuit. Slide 52 52 Magneto Ignition System Parts Magnets Points (Breaker point only) Trigger coil Conductors Spark plug Condenser (Breaker point only) Lamination stack Primary winding Secondary winding Slide 53 53 Magneto Ignition system As magnets in flywheel rotate past the magneto, the points close. The magnetic flux of the magnets in the flywheel induces a current in the primary coil. With current flowing in the primary circuit, a magnetic field develops around the primary coil. This magnetic field also surrounds the secondary coil. As the flywheel continues to rotate the breaker points open. Slide 54 54 Magneto Ignition System- Firing Spark Plug When the breaker points open the magnetic field produced by the current in the primary winding collapses. The collapsing magnetic field flows across the secondary coil which induces a current in the secondary coil. Because there is a 60:1 ratio of windings in the two coils, the voltage is transformed to the 10,000 and 15,000 volts needed to fire the spark plug. Slide 55 55 Magneto Ignition system As long as the flywheel is rotating and the ignition switch is on, the spark plug fires every time the magnets move past the magneto. Slide 56 56 3. Starting System The purpose of the starting system is to use energy to turn the engine until it starts. System components may include: Electrical source Starting motor Conductors Ignition switch Solenoid switch Two primary electrical systems. Single switch Solenoid Slide 57 57 Staring Systems--cont. Single Switch For systems with a single switch the switch must be able to switch the current for the starting motor. Requires a heavy duty switch because starter motors drawn a lot of current. Solenoid In this system the ignition switch only switches the current that powers the solenoid. The solenoid has heavy duty contacts for switching the current to the starting motor. Slide 58 58 4. Accessories Small engines are used on machines that may require electricity to operate accessories. Accessories may include: Lights Electrical clutches Electrical lift systems Radio, etc. The conductors must be sized for the electrical load. Each separate circuit should have overload protection. Slide 59 59 5. Safety It is common for small engines to be used on machines that may have one or more electrical safety systems. These systems are usually designed to stop the engine when activated. The electrical system is used because that is the easiest way to automate an engine stopping system. Safety systems can include: Low oil switch Seat switch Anti after fire solenoid Deck switch Transmission switch Tilt switch Slide 60 60 Questions